Process for manufacturing trench MIS device having implanted drain-drift region and thick bottom oxide

ABSTRACT

A trench MIS device is formed in a P-epitaxial layer that overlies an N-epitaxial layer and an N+ substrate. In one embodiment, the device includes a thick oxide layer at the bottom of the trench and an N-type drain-drift region that extends from the bottom of the trench to the N-epitaxial layer. The thick insulating layer reduces the capacitance between the gate and the drain and therefore improves the ability of the device to operate at high frequencies. Preferably, the drain-drift region is formed at least in part by fabricating spacers on the sidewalls of the trench and implanting an N-type dopant between the sidewall spacers and through the bottom of the trench. The thick bottom oxide layer is formed on the bottom of the trench while the sidewall spacers are still in place. The drain-drift region can be doped more heavily than the conventional “drift region” that is formed in an N-epitaxial layer. Thus, the device has a low on-resistance. The N-epitaxial layer increases the breakdown voltage of the MIS device. In alternative embodiments, the thick bottom oxide layer can be omitted.

This application is a divisional of application Ser. No. 10/454,031,filed Jun. 4, 2003, which is a continuation-in-part of application Ser.No. 10/326,311, which is a continuation-in-part of the followingapplications: application Ser. No. 10/317,568, filed Dec. 12, 2002,which is a continuation-in-part of application Ser. No. 09/898,652,filed Jul. 3, 2001; application Ser. No. 10/176,570, filed Jun. 21,2002; and application Ser. No. 10/106,812, filed Mar. 26, 2002, which isa continuation-in-part of application Ser. No. 09/927,143, filed Aug.10, 2001. Each of the foregoing applications is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

This invention relates to trench-gated power MOSFETs with superioron-resistance and breakdown characteristics and in particular to trenchMOSFETs that are suitable for high frequency operation. This inventionalso relates to a process for manufacturing such a MOSFET.

BACKGROUND OF THE INVENTION

Some metal-insulator-semiconductor (MIS) devices include a gate locatedin a trench that extends downward from the surface of a semiconductorsubstrate (e.g., silicon). The current flow in such devices is primarilyvertical and, as a result, the cells can be more densely packed. Allelse being equal, this increases the current carrying capability andreduces the on-resistance of the device. Devices included in the generalcategory of MIS devices include metal-oxide-semiconductor field effecttransistors (MOSFETs), insulated gate bipolar transistors (IGBTs), andMOS-gated thyristors.

Trench MOSFETs, for example, can be fabricated with a hightransconductance (g_(m,max)) and low specific on resistance (R_(on)),which are important for optimal linear signal amplification andswitching. One of the most important issues for high frequencyoperation, however, is reduction of the MOSFET's internal capacitances.The internal capacitances include the gate-to-drain capacitance(C_(gd)), which is also called the feedback capacitance (C_(rss)), theinput capacitance (C_(iss)), and the output capacitance (C_(oss)).

FIG. 1 is a cross-sectional view of a conventional n-type trench MOSFET10. In MOSFET 10, an n-type epitaxial (“N-epi”) layer 14 is grown on anN⁺ substrate 12. N-epi layer 14 may be a lightly doped layer, that is,an N⁻ layer. A p-type body region 16 separates N-epi layer 14 from N⁺source regions 18. Current flows vertically through a channel (denotedby the dashed lines) along the sidewall of a trench 20. The sidewall andbottom of trench 20 are lined with a thin gate insulator 22 (e.g.,silicon dioxide). Trench 20 is filled with a conductive material, suchas doped polysilicon, which forms a gate 24. Trench 20, including gate24 therein, is covered with an insulating layer 26, which may beborophosphosilicate glass (BPSG). Electrical contact to source regions18 and body region 16 is made with a conductor 28, which is typically ametal or metal alloy. A body contact region 30 facilitates ohmic contactbetween metal 28 and P body 16. Gate 24 is contacted in the thirddimension, outside of the plane of FIG. 1.

A significant disadvantage of MOSFET 10 is a large overlap region formedbetween gate 24 and N-epi layer 14, which subjects a portion of thingate insulator 22 to the drain operating voltage. The large overlaplimits the drain voltage rating of MOSFET 10, presents long termreliability issues for thin gate insulator 22, and greatly increases thegate-to-drain capacitance, C_(gd), of MOSFET 10. In a trench structure,C_(gd) is larger than in conventional lateral devices, limiting theswitching speed of MOSFET 10 and thus its use in high frequencyapplications.

One possible method to address this disadvantage is described inapplication Ser. No. 09/591,179 and is illustrated in FIG. 2. FIG. 2 isa cross-sectional view of a trench MOSFET 40 with an undoped polysiliconplug 42 near the bottom of trench 20. MOSFET 40 is similar to MOSFET 10of FIG. 1, except for polysilicon plug 42, which is isolated from thebottom of trench 20 by oxide layer 22 and from gate 24 by oxide layer44. The sandwich of oxide layer 22, polysilicon plug 42, and oxide layer44 serves to increase the distance between gate 24 and N-epi layer 14,thereby decreasing C_(gd).

In some situations, however, it may be preferable to have a materialthat is a better insulator than undoped polysilicon in the bottom oftrench 19 to minimize C_(gd) for high frequency applications.

One possible method to address this issue is described in applicationSer. No. 09/927,320 and is illustrated in FIG. 3. FIG. 3 is across-sectional view of a trench MOSFET 50 with a thick oxide layer 52near the bottom of trench 20. Thick oxide layer 52 separates gate 24from N-epi layer 14. This circumvents the problems that occur when onlythin gate insulator 15 separates gate 24 from N-epi layer 14 (the drain)as in FIG. 1. Thick oxide layer 52 is a more effective insulator thanpolysilicon plug 42 as shown in FIG. 2, and this decreases thegate-to-drain capacitance, C_(gd), of MOSFET 50 compared to MOSFET 40 ofFIG. 2.

Nonetheless, the solution of FIG. 3 still has a thin gate oxide region54 between body region 16 and thick oxide layer 52. This is because thelower junction of body region 16 and the top edge of thick oxide layer52 are not self-aligned. If body region 16 extends downward past the topedge of thick oxide layer 52, MOSFET 50 could have a high on-resistance,R_(on), and a high threshold voltage. Since this alignment is difficultto control in manufacturing, a substantial margin of error must beallowed to prevent an overlap between body region 16 and thick oxidelayer 52, and this can lead to significant gate-to-drain overlap in thingate oxide region 54. Thin gate region 54 also exists in MOSFET 40 ofFIG. 2, between body region 16 and polysilicon plug 42. Thus, C_(gd) canstill be a problem for high frequency applications. Accordingly, atrench MOSFET with decreased gate-to-drain capacitance, C_(gd), andbetter high frequency performance is needed.

Another problem with trench MIS devices relates to the strength of theelectric field at the corner of the trench, represented, for example, bycorner 56 shown in FIG. 1. The field strength is at a maximum at thecorner of the trench, and therefore this is normally the location atwhich avalanche breakdown occurs. Avalanche breakdown generally leads tothe generation of hot carriers, and when breakdown occurs near the gateoxide layer, the hot carriers may be injected into the gate oxide layer.This can damage or rupture the gate oxide layer and presents long-termreliability problems for the device. It is preferable for breakdown totake place in the bulk silicon, away from the gate oxide layer.

One technique for reducing the strength of the electric field at thecorners of the trench and promoting breakdown in the bulk silicon awayfrom the trench is taught in U.S. Pat. No. 5,072,266. This technique isillustrated in FIG. 4, which shows a MOSFET 60 MOSFET 60 is similar inMOSFET 10 of FIG. 1 except that a deep P+ diffusion 62 extends downwardfrom the P body 16 to a level below the bottom of trench 20. Deep P+diffusion 62 has the effect of shaping the electric field in such a wayas to reduce its strength at the corner 56 of the trench.

While the technique of U.S. Pat. No. 5,072,266 improves the breakdownperformance of the MOSFET, it sets a lower limit on the cell pitch,shown as “d” in FIG. 4, because if the cell pitch is reduced too much,dopant from the deep P+ diffusion will get into the channel region ofthe MOSFET and increase its threshold voltage. Reducing the cell pitchincreases the total perimeter of the cells of the MOSFET, providing agreater gate width for the current, and thereby reduces theon-resistance of the MOSFET. Thus, the net effect of using the techniqueof the Bulucea patent to improve the breakdown characteristics of theMOSFET is that it becomes more difficult to reduce the on-resistance ofthe MOSFET.

To summarize, there is a clear need for an MIS structure that provides alow on-resistance and threshold voltage and yet is capable ofhigh-frequency operation.

SUMMARY OF THE INVENTION

In an MIS device according to this invention, substrate of a firstconductivity type is overlain by an epitaxial (“epi”) layer of a secondconductivity type. A trench is formed in the epi layer, and a gate islocated in the trench, separated from the epi layer by an oxide or otherinsulating layer.

To minimize the gate-to-drain capacitance C_(gd) a thick insulatinglayer, preferably oxide, is formed on the bottom of the trench. Thetrench is lined with a relatively thick layer of, for example, nitride,and the nitride layer is directionally etched to remove the nitridelayer from the bottom of the trench. At this point a dopant of the firstconductivity type is implanted through bottom of the trench to form adrain-drift region extending from the trench bottom to the substrate.

The thick insulating layer can be formed in several ways. An oxide orother insulating layer can be deposited by, for example, chemical vapordeposition (CVD), and the thick insulating layer may be etched backuntil only a “plug” remains on the bottom of the trench. An oxide layermay be thermally grown on the bottom of the trench. A deposition processmay be carried out in such a way that the deposited material (e.g.,oxide) deposits preferentially on the silicon at the bottom of thetrench as opposed to the material (e.g., nitride) that lines thesidewalls of the trench.

After the thick insulating layer has been formed on the bottom of thetrench, the material lining the sidewalls of the trench is removed. Arelatively thin gate oxide layer is formed on the sidewalls of thetrench, and the trench is filled with a conductive gate material such asdoped polysilicon. A threshold adjust or body implant may be performed,and source regions of the first conductivity type are formed at thesurface of the epi layer.

The drain-drift region can be formed in several ways. A dopant of thesecond conductivity type may be implanted through the bottom of thetrench at a dose and energy such that it extends from the trench bottomto the substrate with no diffusion. Alternatively, the dopant of thesecond conductivity type may be implanted through the trench bottom at alower energy such that it initially forms a region of the secondconductivity type just below the trench bottom, and the dopant maydiffused downward to the substrate by subjecting the structure to anelevated temperature for a predetermined period of time. Alternatively,a layer of the second conductivity type may be implanted to a locationat or near the interface between the epi layer and the substrate, andthe dopant may diffused upward to the bottom of the trench. Theforegoing processes may be combined: a region of the second conductivitytype may be formed just below the trench bottom and a layer of thesecond conductivity type may be implanted to a location at or near theinterface between the epi layer and the substrate, and the structuremaybe heated to cause the region and the layer to merge. A series ofimplants may be performed to create drain-drift region that includes a“stack” of second conductivity type regions between the trench bottomand the substrate.

The MIS device that results from this process has a thick oxide or otherinsulating layer at the bottom of the trench and a drain-drift regionthat extends from the bottom of the trench to the substrate. Thejunctions of the drain-drift region are preferably self-aligned with theedges of the thick insulating layer. This minimizes the gate-to-draincapacitance without running the risk of impairing the threshold voltageor on-resistance of the device. At the center of the MOSFET cells, theP-epi layer extends below the level of the trench bottom, assuring thatany breakdown will take place away from the gate oxide layer. There isno deep implant of the kind taught in U.S. Pat. No. 5,072,266, however,so the cell pitch may be set without concern that dopant of the secondconductivity type will get into the channel region and adversely affectthe threshold voltage of the device.

To increase the breakdown voltage of the device, a lightly-doped epilayer of the first conductivity type may be formed on top of thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional trench MOSFET formed in an N epi layeroverlying an N+ substrate.

FIG. 2 shows a trench MOSFET with an undoped polysilicon plug near thebottom of the trench.

FIG. 3 shows a trench MOSFET with a thick oxide layer near the bottom ofthe trench.

FIG. 4 shows a MOSFET with a deep P+ diffusion extending downward to alevel below the bottom of the trench near the center of the cell.

FIG. 5A shows an MIS device in accordance with this invention.

FIG. 5B shows the depletion regions that form in the MIS device of FIG.5A when the device is reverse biased.

FIG. 6 shows an MIS device according to this invention wherein the epilayer is divided into two sublayers having different dopingconcentrations.

FIGS. 7A and 7B are graphs prepared using the computer simulationprogram SUPREME, showing the dopant concentrations in the MOSFET of FIG.5A at vertical cross-sections through the channel region and the bottomof the trench, respectively.

FIGS. 8A and 8B are graphs prepared using the computer simulationprogram MEDICI, showing the dopant concentrations in the MOSFET of FIG.5A at vertical cross-sections through the channel region and the bottomof the trench, respectively.

FIG. 9A is a graph of the doping profile taken at a verticalcross-section through the channel of a conventional MOSFET such as theone shown in FIG. 1 showing that the doping concentration in the channelregion declines rapidly in the direction towards the drain.

FIG. 9B is another graph of the doping profile taken at a verticalcross-section through the channel of a MOSFET illustrating that thedoping concentration in the channel region is relatively constant.

FIGS. 10A and 10B are doping profile graphs similar to the graph of FIG.9B illustrating the addition of a threshold adjust implant and a bodyimplant, respectively.

FIG. 11 shows the general shape of the doping profile in a verticalcross-section below the trench when the drain-drift region is formed byimplanting a deep layer and up-diffusing the deep layer.

FIGS. 12A-12G illustrate a process of forming a drain-drift region byimplanting dopant between trench sidewall spacers and through the bottomof the trench.

FIGS. 12H and 12I illustrate a process of forming a drain-drift regionby implanting dopant between trench sidewall spacers into a regionimmediately below the bottom of the trench and diffusing it downward tothe substrate.

FIGS. 12J and 12K illustrate a process of forming a drain-drift regionby implanting a deep layer of dopant below the trench and diffusing thedopant upward to the trench.

FIGS. 12L and 12M illustrate a process of forming a drain-drift regionby implanting dopant between trench sidewall spacers to form both arelatively shallow region immediately below the bottom of the trench anda deep layer below the trench and then diffusing the dopant until theshallow region and the deep layer merge.

FIG. 12N illustrates a process of forming a drain-drift region byperforming a series of implants at different energies between trenchsidewall spacers and through the bottom of the trench to form a stack ofregions.

FIG. 12O shows an embodiment with a heavily-doped region implanted inthe drain-drift region.

FIGS. 13A-13C illustrate a process of forming a thick bottom oxide layerby depositing an oxide between the trench sidewall spacers.

FIG. 14 illustrates a process of forming a thick bottom oxide layer bythermally growing an oxide between the trench sidewall spacers.

FIGS. 15A-15C illustrate the process of FIG. 14 with sidewall spacers ofvarious thicknesses.

FIGS. 16A and 16B illustrate a process of forming a thick bottom oxidelayer by utilizing the differential deposition rates of oxide on variousmaterials.

FIGS. 17A-17I illustrate a process for continuing the fabrication of anMIS device after the thick bottom oxide layer has been formed.

FIGS. 18A and 18B show an embodiment wherein the epi layer is initiallylightly doped with either N-type or P-type impurity and a P-type isimplanted as a body dopant.

FIGS. 19A and 19B illustrate how the invention simplifies the creationof an edge termination region in an MIS device.

FIG. 20 shows an embodiment wherein the drain-drift region is omittedand the trench extends through the epi layer into the substrate.

FIGS. 21-25 show embodiments in which a lightly doped epi layer of thesame conductivity type as the substrate is formed on the substrate toincrease the breakdown voltage of the device.

FIG. 26 shows a MOSFET similar to the MOSFET shown in FIG. 21, exceptthat the thick bottom oxide has been omitted.

DESCRIPTION OF THE INVENTION

FIG. 5A shows a typical MIS device 70 in accordance with this invention.MIS device 70 is a MOSFET, but it could be another type of MIS device,such as an insulated gate bipolar transistor (IGBT), or a MOS-gatedthyristor.

MIS device 70 is formed in an epitaxial (“epi”) layer 102, which isgenerally doped with P-type impurity and which lies on top of an N+substrate 100. N+ substrate 100, which forms the drain of the device,can have a resistivity of from 5×10⁻⁴ ohm-cm to 5×10⁻³ ohm-cm, forexample, and P-epi layer 102 can be doped with boron to a concentrationof from 1×10¹⁵ cm⁻³ to 5×10¹⁷ cm⁻³. N+ substrate 100 is typically about200 microns thick and epi layer 102 could be from 2 microns to 5 micronsthick.

A trench 110 is formed in P-epi layer 102, the trench 110 being linedwith a gate oxide layer 170 and being filled with polysilicon withserves as a gate 174. An N+ source region 178 and a P+ body contactregion 180 are formed at the surface of P-epi layer 102. The remainingportion of P-epi layer 102 forms a P-type base or body 103. Body 103forms a junction with the N+ substrate 100 that is substantiallycoincident with the interface between the P-epi layer 102 and N+substrate 100.

Electrical contact is made to N+ source region 178 and P+ body contactregion 180 by a metal layer 184. A borophosphosilicate glass (BPSG)layer 182 insulates gate 174 from metal layer 184. Gate 174 is contactedelectrically in the third dimension, outside the plane of the drawing.

In accordance with this invention, the drain of device 70 includes (a)an N-type drain-drift region 116, which extends between the bottom oftrench 110 and N+ substrate 100, and (b) a thick bottom oxide region150, which is formed in the trench 110 adjacent to drain-drift region116. A junction 105 between N drain-drift region 116 and P body 103extends between N+ substrate 100 and trench 110. N drain-drift region116 can be doped, for example, with phosphorus to a concentration offrom 5×10¹⁵ cm⁻³ to 5×10¹⁷ cm⁻³.

FIG. 7A is a graph of the doping concentration in MOSFET 70. The graphwas prepared by the computer simulation program SUPREME and is taken ata vertical section through the channel region, designated I-I in FIG.5A. The curves indicated show the doping concentrations of arsenic andboron, and the third curve shows the net doping concentration. FIG. 7Bis a similar graph taken at a vertical section transecting the bottom ofthe trench designated II-II in FIG. 5A. The horizontal axis of FIG. 7Ais the distance in microns below the surface of the P-epi layer; thehorizontal axis of FIG. 7B is the distance in microns below the bottomof the trench. The vertical axis of FIGS. 7A and 7B is the logarithms₁₀of the doping concentration in atoms/cm⁻³. Note that in FIG. 7A theconcentration of boron, which is the background dopant in P-epi layer102, is relatively flat and dominates in the channel region. The dopingconcentration of arsenic increases as one moves from the channel regioninto the source or the drain.

FIGS. 8A and 8B are graphs of the doping concentration at the samesections, respectively, as FIGS. 7A and 7B. FIGS. 8A and 8B, however,were prepared using the computer simulation program MEDICI and show onlythe net doping concentration whether N-type or P-type.

The SUPREME and MEDICI simulations differ in that SUPREME considers onlythe doping concentrations at a single vertical cross-section, withouttaking into account the effect of dopants at other laterally displacedpositions, while MEDICI takes into account all dopants in thetwo-dimensional plane of the drawing.

The following are among the advantages of MOSFET 70:

1. Avalanche breakdown will generally occur at the interface between theN+ substrate 100 and the P-epi layer 102, away from the trench (e.g., atthe location designated 72 in FIG. 5A). This avoids damage to the gateoxide layer 170 from the hot carriers generated in the area of thebreakdown.

2. The gate oxide 170 at the corners of the trench, where the electricfield reaches a maximum, is protected from rupture.

3. A higher punchthrough breakdown can be obtained for a given thresholdvoltage. The junctions 105 between the N drain-drift region 116 and theP body 103 extend downward to the N+ substrate 100. As shown in FIG. 5B,when PN junctions 105 are reverse-biased, as they are when MOSFET 70 isin an off condition and is blocking current, the depletion regions,denoted by the dashed lines 105A, 105B, extend along the entire lengthof junctions 105, and as a result the depletion region in the area ofthe channel does not expand as quickly towards the source region. Theexpansion of the depletion regions towards the source region is thecondition that causes punchthrough breakdown.

4. Moreover, a higher punchthrough breakdown voltage can be obtained fora given threshold voltage. As shown in FIG. 9A, in a conventional MOSFEThaving a diffused body, the dopant concentration of the body falls offrapidly as one approaches the N-epi (drift region). The thresholdvoltage is determined by the peak doping concentration N_(A peak). Thepunchthrough breakdown voltage is determined by the total amount ofcharge Q_(channel) in the channel region (represented by the area underthe P body curve in FIG. 9A). In a MOSFET of this invention the dopingprofile of the P body region is relatively flat, as shown in FIG. 9B.Therefore, N_(A peak) can be the same while the total charge in thechannel is greater, providing a higher punchthrough breakdown voltage.

5. Since there is no deep body diffusion in each cell (of the kindtaught in U.S. Pat. No. 5,072,266) the cell pitch can be reduced withoutconcern that additional P-type dopant will get into the channel region,raising the threshold voltage of the MOSFET. Thus the cell packingdensity can be increased. This reduces the on-resistance of the device.

6. In a conventional trench MOSFET a lightly-doped “drift region” isoften formed between the channel and the heavily-doped substrate. Thedoping concentration in the drift region must be kept below a certainlevel. Otherwise effective depletion is not obtained and the strength ofthe electric field at the corner of the trench becomes too great.Keeping the doping concentration in the drift region low, however,increases the on-resistance of the device. In contrast, the Ndrain-drift region 116 of this invention can be doped more heavilybecause the shape of N drain-drift region 116 and the length of junction105 between N drain-drift region 116 and P body region 103 provide moreeffective depletion. A more heavily doped N drain-drift region 116reduces the on-resistance of the device.

7. As shown in FIG. 19A, there is no need for a separate P-typediffusion in the termination region of the MOSFET, since P-epi layer 102extends to N+ substrate 100 except where the N drain-drift regions 116are located. FIG. 19B shows the termination region of a conventionalMOSFET which includes a P-type diffusion 75. The elimination of theP-type termination diffusion or field ring reduces the number of maskingsteps. For example, in the process described herein only five maskingsteps are required.

Formation of Drain-Drift Region

FIGS. 12A-12N are cross-sectional views illustrating one embodiment of aprocess for fabricating a trench MOSFET, such as MOSFET 70 of FIG. 5A,in accordance with the present invention. As shown in FIG. 12A, theprocess begins with a lightly-doped P-epi layer 102 (typically about 6to 8 μm thick) grown on a heavily doped N+ substrate 100. A pad oxide104 (e.g., 100-200 Å thick) is thermally grown by dry oxidation at 950°C. for 10 minutes on P-epi layer 102. As shown in FIG. 12B, a nitridelayer 106 (e.g., 200-300 Å thick) is deposited by chemical vapordeposition (CVD) on pad oxide 104. Using a normal photolithographicprocess and a first (trench) mask, nitride layer 106 and pad oxide 104are patterned to form an opening 108 where a trench is to be located. Asshown in FIG. 12C, a trench 110 is etched through opening 108, typicallyusing a dry plasma etch, for example, a reactive ion etch (RIE). Trench110 may be about 0.5-1.2 μm wide and about 1-2 μm deep.

A second pad oxide 112 (e.g., 100-200 Å) is thermally grown on thesidewall and bottom of trench 110, as shown in FIG. 12D. A thick nitridelayer 114 (e.g., 1000-2000 Å) is deposited conformally by CVD on thesidewall and bottom of trench 110 as well as on top of nitride layer106, as shown in FIG. 12E. Nitride layer 114 is etched using adirectional, dry plasma etch, such as an RIE, using an etchant that hasa high selectivity for nitride layer 114 over oxide. The nitride etchleaves spacers 115 of nitride layer 114 along the sidewalls of trench110, while exposing pad oxide 112 at the central bottom portion oftrench 110, as shown in FIG. 12F. It is possible that nitride layer 114may be overetched to such a degree that nitride layer 106 is removedfrom the top of pad oxide 104.

Leaving sidewall spacers 115 in place, an N-type dopant is implantedthrough the pad oxide 112 at the bottom of trench 110 to produce Ndrain-drift region 116 (FIG. 12G). For example, phosphorus can beimplanted at a dose of 1×10¹³ cm⁻² to 1×10¹⁴ cm⁻² and an energy of 300keV to 3.0 MeV. To avoid significant diffusion of the phosphorus and theconsequent expansion of N drain-drift region 116, the thermal budget towhich the structure is thereafter exposed is limited to the equivalentof about 950° C. for 60 minutes, or the structure can be subjected to arapid thermal anneal (RTA) at 1050° C. for 90 seconds. In either case, Ndrain-drift region 116 retains essentially the compact shape shown inFIG. 12G. Advantageously, in the cross-sectional view of FIG. 12G, atleast 75% and preferably 90% of the N drain-drift region 116 is locateddirectly below the trench 110.

Alternatively, N drain-drift region 116 can be formed by implanting thephosphorus at a lower energy of 30 keV to 300 keV (typically 150 keV) toform an N-type region 118 immediately below the trench (FIG. 12H), andthen diffusing the phosphorus by heating at 1050° C. to 1150° C. for 10minutes to 120 minutes (typically 1100° C. for 90 minutes), so thatN-type region 118 expands downward and laterally to form a drain-driftregion 120 having a shape of the kind shown in FIG. 12I.

In another variant of the process, a deep layer 122 (e.g., phosphorus)is implanted at a relatively high energy to a location below the trench,as shown in FIG. 12J, and a thermal process is used to up-diffuse thephosphorus until it reaches the bottom of the trench, yielding adrain-drift region 124, as shown in FIG. 12K. This is distinguishablefrom the process described above in conjunction with FIG. 12G, whereafter the implant the N-type dopant extends from the bottom of trench110 to the interface between the N+ substrate and the P-epi layer, or inconjunction with FIG. 12H, where after the implant the dopant lies justbelow the bottom of the trench. When the N-type dopant is implanted at arelatively high energy to form deep layer 122, variations in the depthof the trench, the thickness of the P-epi layer 102, and the implantenergy may cause layer 122 to be located either above the interfacebetween N+ substrate 100 and P-epi layer 102 (e.g., if P-epi layer 102is thick and/or the trench depth is small) or in N+ substrate 100 (e.g.,if P-epi layer 102 is thin and/or the trench depth is large).

FIG. 11 shows the general shape of the doping profile in a verticalcross-section starting at the bottom of the trench when the drain-driftregion is formed by up-diffusing a deep implanted layer. As indicated,the concentration of N-type dopant in the drain-drift region increasesmonotonically with increasing distance below the bottom of the trench.This is distinguishable from the doping profile below the trench in aMOSFET formed using the low-energy process, as shown in FIG. 8B, wherethe doping concentration initially decreases and then increases in thevicinity of the N+ substrate.

Using the process illustrated in FIGS. 12J and 12K provides an Ndrain-drift region that is confined largely to the area directly belowthe trench and allows a smaller cell pitch. The process is also easierto control and provides greater throughput.

Alternatively, a combination up-diffusion, down-diffusion process can beused to form the drain-drift region. As shown in FIG. 12L, deep N layer122 (e.g., phosphorus) is formed at the interface of N+ substrate 102and P epi layer 100 by a high-energy implant process. As described abovein connection with FIG. 12H, an N-type dopant is implanted through thebottom of the trench to form N+ region 118 beneath the trench. Thestructure is then heated, for example, to 900 to 1100° C. Deep N layer122 diffuses upward and N region 118 diffuses downward until they merge,forming N-type drain-drift region 126, as shown in FIG. 12M.

Yet another alternative is to form the drain-drift region with a seriesof three or more N implants at successively greater energies to form astack of overlapping implanted regions 128 as shown in FIG. 12N. Thestack 128 includes four implanted regions 128A-128D, but fewer or morethan four implants could also be used to form the stack. The stack couldbe formed with essentially no diffusions (i.e., no heating), or it couldbe heated to diffuse the dopant and increase the amount of overlapbetween the regions 128A-128D.

Optionally, to increase current spreading in the drain-drift region andfurther reduce the on-resistance of the device, a heavily-doped N+region 130 can be implanted in the drain-drift region 116, as shown inFIG. 12O.

At the conclusion of the process, whether high energy or low energy, theN drain-drift region extends from the N+ substrate to the bottom of thetrench. In many cases, the junction between the N drain-drift region andthe P-epi layer extends from the substrate to a sidewall of the trench.If the low energy implant process is used and the dopant is laterthermally diffused, the junction between the drain-drift region and theP-epi layer takes the form of an arc that is concave towards theinterior of drain-drift region (FIG. 12I).

Any of the methods described above may be used to form the drain-driftregion. In the following explanation of how a thick bottom insulatinglayer is formed, it will be assumed that the implant process representedby FIG. 12G is used. It should be understood, however, that any of thealternative methods could be used as well.

Formation of Thick Bottom Oxide

The process begins, as shown in FIG. 13A, with the deposition of a thickinsulating layer 150, which may be 2-4 μm thick, for example. Thedeposition process is chosen to be non-conformal, filling trench 110 andoverflowing onto the top surface of P-epi layer 102. Thick insulatinglayer 150 may be, for example, a low temperature oxide (LTO), a chemicalvapor deposition (CVD) oxide, a phosphosilicate glass (PSG), aborophosphosilicate glass (BPSG), or another insulating material. In thefollowing description, insulating layer 150 is assumed to be a CVD oxidelayer.

Oxide layer 150 is etched back into trench 110, typically by performinga wet etch with an etchant that has high selectivity for oxide overnitride. Oxide layer 150 is etched until only about 0.1-0.2 μm remainsin trench 110, as shown in FIG. 13B forming a thick bottom oxide layer151.

Nitride layer 106 and spacers 115 are removed, typically by performing awet etch with an etchant that has high selectivity for nitride overoxide is. Pad oxide 104 and the exposed portion of pad oxide 112,typically by a wet etch. This wet etch removes a small but insignificantportion of thick oxide layer 151. The resulting structure is shown inFIG. 13C, with thick oxide layer 151 remaining at the bottom of trench110.

In another variation according to this invention, a gradual transitionis formed between between the thick and thin sections of the gate oxidelayer.

The process may be identical to that described above through the stepillustrated in FIG. 12F, where the nitride etch leaves sidewall spacers115 along the sidewalls of trench 110, while exposing pad oxide 112 inthe central bottom portion of trench 110. In the next step, however,instead of depositing a thick insulating layer, a thick oxide layer isgrown by a thermal process. When this is done, the thermal oxideconsumes part of the silicon and thereby undercuts the edges of sidewallspacers 115, causing the nitride to “lift off” of the surface of thetrench. This forms a structure that is similar to the “bird's beak” in aconventional LOCOS (LOCal Oxidation of Silicon) process that is oftenused to create field oxide regions on the top surface of a semiconductordevice.

FIG. 14 shows the structure after a thermal oxide layer 158 has beengrown at the bottom of trench 110. The structure is shown in detail inFIG. 15A. The edges of thermal oxide layer 158 have pushed undersidewall spacers 115 and as a result become sloped or tapered.

Altering the thickness of the sidewall spacers allows one to positionthe edges of the oxide layer at different locations. FIG. 15A showsrelatively thick sidewall spacers 115, and as a result the edges ofoxide layer 158 are located on the bottom of trench 110. FIG. 15B showsthinner sidewall spacers 115A, with the edges of oxide layer 158Alocated essentially at the corners of trench 110. FIG. 15C shows eventhinner sidewall spacers 115B, with the edges of oxide layer 158Blocated on the sidewalls of trench 110.

In a similar manner, the edges of the oxide layer may be positioned atvarious intermediate points by altering the thickness of the sidewallspacers. The thickness of the sidewall spacers is independent of thewidth or depth of trench. For example, if the sidewall spacers are inthe range of 1,500 to 2,000 Å thick, the edges of the oxide layer wouldmost likely be located on the bottom of the trench (FIG. 15A). If thesidewall spacers are 500 Å or less thick, the edges of the oxide layerwould typically be located on the sidewalls of trench (FIG. 15C).

The oxide layer may be grown, for example, by heating the siliconstructure at a temperature from 1,000° C. to 1,200° C. for 20 minutes toone hour.

Yet another way of forming a thick bottom oxide is illustrated in FIGS.16A and 16B. After drain-drift region 116 and sidewall spacers 115 havebeen formed, as described above and shown in FIGS. 12A-12G, an oxidelayer 160 is deposited by a process that causes it to depositselectively on the silicon exposed in the bottom of trench 110 ratherthan on the sidewall spacers 115. One process that may be used is asubatmospheric chemical vapor deposition (SACVD) process, using ozone todrive the chemical reaction. During the reaction, the ozone readilydissociates to release atomic oxygen, which combines with a precursorsuch as TEOS to form silicon dioxide. The structure may then beannealed.

Table 1 illustrates exemplary process parameters for ozone-activatedTEOS SACVD formation of thick insulating layer 21.

TABLE 1 Temperature  400° C. Pressure  600 Torr Ozone flow rate 5000sccm Helium flow rate 4000 sccm TEOS flow rate  325 mgm GDP-to-waferspacing  250 mm

Spacers 115 may include materials other than nitride. The material usedfor the spacers is selected such that silicon dioxide preferentiallydeposits on silicon over the spacers. The selection of the material forthe spacers depends on the oxide deposition process used. Table 2illustrates the deposition selectivity of several materials duringozone-activated TEOS SACVD.

TABLE 2 Material Deposition Selectivity Si:Nitride 5:1 Si:Thermal Oxide3:1 Si:TEOS PECVD Oxide 2:1 Si:SiH₄ PECVD Oxide 1:1 Si:PECVD BPSG 1:1

As shown in Table 2, during ozone-activated TEOS SACVD, silicon dioxidedeposits on silicon five times faster than it deposits on nitride. Thus,during fabrication of a device using nitride sidewall spacers 115, thesilicon dioxide deposited in the bottom of trench 110 would be aboutfive times thicker than any silicon dioxide deposited on the nitridesidewall spacers 115. In fact, for 3000 Å of oxide film growth on thesilicon surface, no oxide growth was observed on the nitride surface.The deposition selectivity is possibly due to the lower surface energyof silicon nitride compared to silicon. As illustrated in Table 2,thermally grown silicon dioxide or TEOS PECVD deposited silicon dioxidemay also make a suitable material for the spacers when the deposition oflayer 160 is ozone-activated TEOS SACVD, since silicon dioxide will alsopreferentially deposit on silicon over these materials. SiH₄ PECVDdeposited silicon dioxide or PECVD deposited BPSG would not makesuitable spacer materials for ozone-activated TEOS SACVD, since silicondioxide does not prefer silicon to these materials. If a depositionprocess besides ozone-activated TEOS SACVD is used, materials other thanthose shown in Table 2 may be used for the side wall spacers.

After oxide layer 160 has been deposited, a buffered oxide etch is usedto remove any oxide that deposited on the surfaces of nitride sidewallspacers 115, and a wet nitride etch is used to remove nitride sidewallspacers 115 and nitride layer 106. To ensure that all of the nitride isremoved, another anneal may be performed, for example, at 1,000° C. for5-10 minutes to oxidize any remaining nitride, and the anneal may befollowed by an oxide etch. The oxide etch removes any oxidized nitridebut does not remove significant portions of oxide layer 160.

Pad oxides 104, 112 are also removed, typically by a wet etch. This wetetch removes a small but insignificant portion of oxide layer 160. Theresulting structure is shown in FIG. 16B, with a portion of oxide layer160 left remaining at the bottom of trench 110.

Completion of Device

After the thick bottom oxide has been formed by one of the foregoingprocesses, a sacrificial oxide layer (not shown) can be grown in thesidewalls of the trench and removed. This aids in removing any crystaldamage caused during the etching of the trench. The sacrificial oxidelayer can be approximately 500Å thick and can be thermally grown, forexample, by dry oxidation at 1050° C. for 20 minutes, and removed by awet etch. The wet etch of the sacrificial gate oxide is kept short tominimize etching of oxide layer at the bottom of the trench.

Next, as shown in FIG. 17A, a gate oxide layer 170 or other insulatinglayer (e.g., about 300-1000 Å thick) is formed on the sidewall of trench110 and the top surface of P-epi layer 102. For example, gate oxidelayer 170 may be thermally grown using a dry oxidation at 1050° C. for20 minutes.

As shown in FIG. 17B, a layer 172 of polysilicon or another conductivematerial is deposited (for example, by a low pressure CVD (LPCVD)process) to fill trench 110 and overflow the horizontal surface of oxidelayer 170. Polysilicon layer 172 may be, for example, an in-situ dopedpolysilicon, or an undoped polysilicon layer that is subsequentlyimplanted and annealed, or an alternative conductive material.Polysilicon layer 172 is etched, typically using a reactive ion etch,until the top surface of polysilicon layer 172 is approximately levelwith the top of P-epi layer 102, thereby forming a gate 174, as shown inFIG. 17C. In an N-type MOSFET, gate 174 may be, for example, apolysilicon layer doped with phosphorus to a concentration of 1×10¹⁹cm⁻³. In some embodiments, polysilicon layer 172 may be etched past thetop of trench 110, thereby recessing gate 174 to minimize thegate-to-source overlap capacitance, and an oxide or other insulatinglayer may be formed over gate 174. In many cases, polysilicon layer 172is etched through an opening in a second (gate poly) mask that allows aportion of the polysilicon layer 172 to remain in place where the gate174 is to be contacted by a gate metal portion of metal layer 184 (seeFIG. 17I)

Optionally, if the threshold voltage is to be adjusted, a thresholdadjust implant may be performed, for example, by implanting boronthrough the surface of P-epi layer 102. The boron may be implanted at adose of 5×10¹² cm⁻² and at an energy of 150 keV, yielding aconcentration of P-type atoms of 1×10¹⁷ cm⁻³ in the portion of P-epilayer 102 which will form the channel of the MOSFET. As described above,FIG. 10A shows the dopant profile at a vertical cross-section takenthrough the channel, showing a threshold adjust implant. As shown, thethreshold adjust implant is typically located in an area of the channeljust below the source region. The threshold voltage of the MOSFET isdetermined by the peak doping concentration N_(A peak) of the thresholdadjust implant. If the threshold voltage of the device does not need tobe adjusted, this step can be omitted.

If desired, a P-type dopant such as boron may be implanted to form abody region 176 as shown in FIG. 17D. The doping profile of a typicalbody implant is illustrated in the graph of FIG. 10B. The body implantis somewhat similar to the threshold adjust implant but the energy usedis higher and as a result the body implant extends to a level nearer thejunction between the P-epi layer and the N drain-drift region. Thethreshold voltage of the MOSFET is determined by the peak dopingconcentration N_(A peak) of the body implant. Alternatively, the P bodyimplant may be driven to a level below the bottom of trench 110 butabove the interface between P-epi layer 102 and N+ substrate 100, asshown by body region 186 in FIG. 17E.

Next, the top surface of P-epi layer 102 may be masked with a third(source) mask 190 and an N-type dopant such as phosphorus may beimplanted to form N+ source regions 178, shown in FIG. 17F. Source mask190 is removed. A BPSG layer 182 is deposited on the top surface of thedevice and a fourth (contact) mask 183 is deposited and etched on thesurface of BPSG layer 182, as shown in FIG. 17G. BPSG layer 182 isetched through the openings in contact mask 183, and a P-type dopant isimplanted through the resulting openings in BPSG layer 182 to form P+body contact regions 180, as shown in FIG. 17H. For example, N+ sourceregions 178 can be implanted with arsenic at a dose of 5×10¹⁵ cm⁻² andan energy of 80 keV, yielding a concentration of 1×10²⁰ cm⁻³; P+ bodycontact regions 180 can be implanted with boron at a dose of 1×10¹⁵ cm⁻²and an energy of 60 keV, yielding a dopant concentration of 5×10¹⁹ cm⁻³.

A metal layer 184, preferably aluminum, is deposited as shown in FIG.17I, establishing a short between source regions 178 and body contactregions 180. A fifth (metal) mask (not shown) is used to pattern andetch metal layer 184 into a source metal portion, shown in FIG. 17I, anda gate metal portion that is used to establish electrical contact to thegate. This completes the fabrication of MOSFET 70.

In another embodiment, the epi layer is initially lightly doped witheither N-type or P-type impurity, and a P-type impurity such as boron isimplanted as a body dopant and is driven in until the dopant reaches theinterface between the epi layer and the substrate. Such an embodiment isillustrated in FIGS. 18A and 18B. As shown in FIG. 18B, when the boronhas been implanted and diffused, a P body region is formed on the N+substrate 102.

The structures containing P body 176 as shown in FIG. 17D, P body 186shown in FIG. 17E, and P body 104 as shown in FIG. 18B, can be used inconjunction with any of the processes for forming a drain-drift regiondescribed herein. That includes the process shown in FIGS. 12J and 12K,involving the up-diffusion of a deep implanted layer; the process shownin FIGS. 12L and 12M, involving the up-diffusion of a deep implantedlayer and down diffusion of an implanted region below the bottom of thetrench; and the process shown in FIG. 12N, involving the implanting ofmultiple N-type regions at different energies to form a stack ofoverlapping regions.

FIG. 6 shows an alternative embodiment. In MOSFET 95 the P-epi layer isdivided into sublayers Pepi1 and Pepi2. Using a well-known process, anepi layer having sublayers can be formed by varying the flow rate of thedopant gas while the epi layer is being grown. Alternatively, sublayerPepi1 can be formed by implanting dopant into the upper portion of theepi layer.

The dopant concentration of sublayer Pepi1 can be either greater than orless than the dopant concentration of sublayer Pepi2. The thresholdvoltage and punchthrough breakdown of the MOSFET are a function of thedoping concentration of sublayer Pepi1, while the breakdown voltage andon-resistance of the MOSFET are a function of the doping concentrationof sublayer Pepi2. Thus, in a MOSFET of this embodiment the thresholdvoltage and punchthrough breakdown voltage can be designed independentlyof the avalanche breakdown voltage and on-resistance. The P-epi layermay include more than two sublayers having different dopingconcentrations.

MOSFET 95 includes a gate electrode 202 that is positioned in a trench204, which is lined with an oxide layer. The upper surface of gate 202is recessed into trench 204. The oxide layer includes a thick section206, formed in accordance with this invention, which is locatedgenerally at the bottom of trench 204, and relatively thin sections 210adjacent the sidewalls of trench 204. Between thick section 206 and thinsections 210 are transition regions 208, where the thickness of theoxide layer decreases gradually from thick section 206 to thin sections210. MOSFET 100 also includes PN junctions, which intersect trench 204in the transition regions 208. As described above, the location oftransition regions 208 can be varied by altering the thickness of thenitride layer during the fabrication of MOSFET 95.

MOSFET 95 also includes N+ source regions 214, P+ body contact regions216, a thick oxide layer 218 overlying gate electrode 202, and a metallayer 220 that makes electrical contact with N+ source regions 214 andP+ body contact regions 216. As shown by the dashed lines, MOSFET 95contains a highly doped region 222 at the bottom of trench 204. Highlydoped region 222 may be created by implanting an N-type dopant, such asarsenic or phosphorous, after the nitride layer has been etched as shownin FIG. 12O.

FIG. 20 shows another alternative embodiment. In MOSFET 98 a drain-driftregion is omitted, and trench 230 extends entirely through P-epi layer102 into N+ substrate 100. This embodiment is particularly suitable forlow-voltage (e.g., 5 V or less) MOSFETs.

In order to increase the breakdown voltage of the device, alightly-doped N-type epi layer can be grown on top of the N+ substrate100, underneath the P-epi layer 102. Several embodiments of thisstructure are shown in FIGS. 21-25.

FIG. 21 shows a MOSFET 250 which is similar to MOSFET 70 shown in FIG.5A, except that an N-epi layer 252 has been grown on top of N+ substrate100. N-epi layer 252 could be from 1 to 50 μm thick and could be dopedwith phosphorus to a concentration of from 1×10¹⁵/cm⁻³ to 1×10⁻¹⁷/cm⁻³.The doping concentration of N-epi layer 252 may be either higher orlower than the doping concentration of P-epi layer 102.

Apart from the growth of N-epi layer 252, the process of fabricatingMOSFET 250 is similar to the process of fabricating MOSFET 70, describedabove in conjunction with FIGS. 12A-12G. In particular, as shown in FIG.12G, phosphorus may be implanted through the bottom of the trench toform drain-drift region 116. The energy and dose of the phosphorusimplant are set, however, to ensure that drain-drift region 116 extendsdownward to the upper boundary of N-epi layer 252 rather than to theupper boundary of N+ substrate 100.

FIG. 22 shows a MOSFET 260 which has a drain-drift region 120 similar todrain drift region 120 shown in FIG. 12I. MOSFET 260 is formed byimplanting the phosphorus to form an N-type region immediately below thetrench (see FIG. 12H), and then diffusing the phosphorus by heating sothat the N-type region expands downward and laterally to formdrain-drift region 120 shown in FIG. 22.

FIG. 23 shows a MOSFET 270 which has a drain-drift region 124 is similarto drain-drift region 124 shown in FIG. 12K. MOSFET 270 is formed byimplanting the phosphorus to form an N-type region near the interfacebetween N-epi layer 252 and P-epi layer 102 (see FIG. 12J), and thendiffusing the phosphorus by heating so that the N-type region expandsupward and laterally to form drain-drift region 124 shown in FIG. 23.

FIG. 24 shows a MOSFET 280 which has a drain-drift region 126 similar todrain-drift region 126 shown in FIG. 12M. To fabricate MOSFET 280, adeep N layer (e.g., phosphorus) is formed at the interface of N-epilayer 252 and P epi layer 100 by a high-energy implant process. AnN-type dopant is implanted through the bottom of the trench to form asecond N region immediately beneath the trench. The structure is thenheated, for example, to 900 to 1100° C. The deep N layer diffuses upwardand the second N region diffuses downward until they merge, formingN-type drain-drift region 126, as shown in FIG. 24.

FIG. 25 shows a MOSFET 290 containing a drain-drift region formed of aseries of N implants performed at successively greater energies tocreate a stack of overlapping implanted regions 128, similar to thestructure shown in FIG. 12N. The stack 128 includes four implantedregions, but fewer or more than four implants could also be used to formthe stack. The stack could be formed with no significant diffusions(i.e., no heating), or it could be heated to diffuse the dopant andincrease the amount of overlap between the implanted regions.

Another group of embodiments are similar to those shown in FIGS. 21-25except that the thick bottom oxide region 150 is omitted, and the bottomof the trench is lined with an oxide layer having substantially the samethickness as the oxide layer 170 that lines the walls of trench 110. Tofabricate devices of this kind, an N-type dopant such as phosphorus isimplanted through the bottom of trench 110 at the stage of the processshown in FIG. 12C, and the deposition of nitride layer 114 and theformation of sidewall spacers 115, shown in FIGS. 12E and 12F, areomitted. If the N-type dopant is implanted so as to extend downward fromthe bottom of the trench, as shown in FIG. 12G, a MOSFET 300, shown inFIG. 26, results. Alternatively, a drain-drift region of the kind shownin FIGS. 12H-12I, 12J-12K, 12L-12M, and 12N can be fabricated byfollowing the processes described in connection with those figures. Inall cases the drain-drift region extends from the bottom of trench 110to the junction of N-epi layer 252.

While several specific embodiments of this invention have beendescribed, these embodiments are illustrative only. It will beunderstood by those skilled in the art that numerous additionalembodiments may be fabricated in accordance with the broad principles ofthis invention. For example, while the embodiments described above areN-channel MOSFETs, a P-channel MOSFET may be fabricated in accordancewith this invention by reversing the conductivities of the variousregions in the MOSFET.

1. A process of fabricating a trench MIS device comprising; providing asubstrate of a first conductivity type; forming a first epitaxial layeron the substrate, the first epitaxial layer being doped with a dopant ofthe first conductivity type to a doping concentration that is less thanthe doping concentration of the substrate; forming a second epitaxiallayer on the first epitaxial layer, the second epitaxial layer beinggenerally of a second conductivity type; forming a trench in the secondepitaxial layer, the trench having sidewalls and a bottom; depositing aninsulating layer conformally in the trench and directionally etching theinsulating layer so as to remove a portion of the insulating layer fromthe bottom of the trench, leaving the bottom of the trench intact andleaving sidewall spacers adjacent the sidewalls of the trench;implanting a dopant of the first conductivity type between the sidewallspacers and through the intact bottom of the trench at a dose and energysuch that following the implant the dopant forms a deep layersubstantially separated from the trench; heating the first epitaxiallayer so as to diffuse the dopant upward, thereby forming a drain-driftregion extending between the bottom of the trench and the firstepitaxial layer; forming a bottom insulating layer on the intact bottomof the trench between the sidewall spacers; removing the sidewallspacers; forming a gate insulating layer on a sidewall of the trench,the gate insulating layer being thinner than the bottom insulatinglayer; introducing a conductive material into the trench; implantingdopant of the first conductivity type into the second epitaxial layer toform a source region adjacent the sidewall of the trench and a topsurface of the second epitaxial layer; implanting dopant of the secondconductivity type into the second epitaxial layer to form a body contactregion adjacent the top surface of the second epitaxial layer; forming athird insulating layer over the conductive material in the trench; anddepositing a metal layer, the metal layer being in electrical contactwith the source region and the body contact region, the metal layerbeing electrically insulated from the conductive material in the trenchby the third insulating layer.
 2. The process of claim 1 wherein forminga trench comprises etching the second epitaxial layer and forming a padoxide layer.
 3. The process of claim 1 wherein the insulating layercomprises nitride.
 4. The process of claim 1 wherein forming a bottominsulating layer comprises depositing a layer and etching the layer toform the bottom insulating layer.
 5. The process of claim 4 whereindepositing a layer comprises depositing an oxide layer.
 6. The processof claim 5 wherein depositing a layer comprises depositing a layer bychemical vapor deposition.
 7. The process of claim 4 wherein the bottominsulating layer is a low-temperature oxide layer.
 8. The process ofclaim 4 wherein depositing a layer comprises depositing a glass layer.9. The process of claim 1 wherein forming a bottom insulating layercomprises thermally growing an oxide layer on the bottom of the trench.10. The process of claim 1 wherein forming a bottom insulating layercomprises depositing a material that deposits preferentially on thebottom of the trench as compared with the sidewall spacers.
 11. Aprocess of fabricating a trench MIS device comprising: providing asubstrate of a first conductivity type; forming a first epitaxial layeron the substrate, the first epitaxial layer being doped with a dopant ofthe first conductivity type to a doping concentration that is less thanthe doping concentration of the substrate; forming a second epitaxiallayer on the first epitaxial layer, the second epitaxial layer beinggenerally of a second conductivity type; forming a trench in the secondepitaxial layer, the trench having sidewalls and a bottom; depositing aninsulating layer conformally in the trench and directionally etching theinsulating layer so as to remove a portion of the insulating layer fromthe bottom of the trench, leaving the bottom of the trench intact andleaving sidewall spacers adjacent the sidewalls of the trench;implanting a first portion of a dopant of the first conductivity typebetween the sidewall spacers and through the intact bottom of the trenchat a dose and energy such that following the implant the first portionof dopant forms a region of the first conductivity type located belowthe bottom of the trench and not extending to the first epitaxial layer;implanting a second portion of the dopant between the sidewall spacersand through the intact bottom of the trench at a dose and energy suchthat following the implant the second portion of the dopant forms a deeplayer substantially separated from the trench; heating the firstepitaxial layer so as to diffuse the first portion of dopant downwardand to diffuse the second portion of dopant upward such that the firstand second portions merge, thereby forming a drain-drift regionextending between the bottom of the trench and the first epitaxiallayer; forming a bottom insulating layer on the intact bottom of thetrench between the sidewall spacers; removing the sidewall spacers;forming a gate insulating layer on a sidewall of the trench, the gateinsulating layer being thinner than the bottom insulating layer;introducing a conductive material into the trench; implanting dopant ofthe first conductivity type into the second epitaxial layer to form asource region adjacent the sidewall of the trench and a top surface ofthe second epitaxial layer; implanting dopant of the second conductivitytype into the second epitaxial layer to form a body contact regionadjacent the top surface of the second epitaxial layer; forming a thirdinsulating layer over the conductive material in the trench; anddepositing a metal layer, the metal layer being in electrical contactwith the source region and the body contact region, the metal layerbeing electrically insulated from the conductive material in the trenchby the third insulating layer.
 12. A process of fabricating a trench MISdevice comprising: providing a substrate of a first conductivity type;forming a first epitaxial layer on the substrate, the first epitaxiallayer being doped with a dopant of the first conductivity type to adoping concentration that is less than the doping concentration of thesubstrate; forming a second epitaxial layer on the first epitaxiallayer, the second epitaxial layer being generally of a secondconductivity type; forming a trench in the second epitaxial layer;forming sidewall spacers in the trench; implanting a dopant of the firstconductivity type between the sidewall spacers and through a bottom ofthe trench at a dose and energy such that following the implant thedopant forms a deep layer substantially separated from the trench;heating the first epitaxial layer so as to diffuse the dopant upward,thereby forming a drain-drift region extending between the bottom of thetrench and the first epitaxial layer; forming a bottom insulating layeron the bottom of the trench between the sidewall spacers; removing thesidewall spacers; forming a gate insulating layer on a sidewall of thetrench, the gate insulating layer being thinner than the bottominsulating layer; and introducing a conductive material into the trench.13. A process of fabricating a trench MIS device comprising: providing asubstrate of a first conductivity type; forming a first epitaxial layeron the substrate, the first epitaxial layer being doped with a dopant ofthe first conductivity type to a doping concentration that is less thanthe doping concentration of the substrate; forming a second epitaxiallayer on the first epitaxial layer, the second epitaxial layer beinggenerally of a second conductivity type; forming a trench in the secondepitaxial layer; forming sidewall spacers in the trench; implanting afirst portion of a dopant of the first conductivity type between thesidewall spacers and through a bottom of the trench at a dose and energysuch that following the implant the first portion of dopant forms aregion of the first conductivity type located below the bottom of thetrench and not extending to the first epitaxial layer; implanting asecond portion of the dopant between the sidewall spacers and through abottom of the trench at a dose and energy such that following theimplant the second portion of the dopant forms a deep layersubstantially separated from the trench; heating the first epitaxiallayer so as to diffuse the first portion of dopant downward and todiffuse the second portion of dopant upward such that the first andsecond portions merge, thereby forming a drain-drift region extendingbetween the bottom of the trench and the first epitaxial layer; forminga bottom insulating layer on the bottom of the trench between thesidewall spacers; removing the sidewall spacers; forming a gateinsulating layer on a sidewall of the trench, the gate insulating layerbeing thinner than the bottom insulating layer; and introducing aconductive material into the trench.